1. Field of the Invention
The present invention is directed generally to a metallized film capacitor and, in particular, to a metallized film series section capacitor.
2. Description of the Prior Art
Metallized film capacitors typically include sheets of dielectric material each having a conductive layer thereon, usually a metal, which forms the capacitor electrodes. The conductive layers are generally applied to only one side of the dielectric and the conductive layers on a dielectric are placed against the uncoated side of an adjacent dielectric. The conductive layers are then electrically insulated from each other by the dielectric which holds a charge.
A series section metallized film capacitor includes a center margin electrode and a floating electrode, as shown in FIG. 1. The center margin electrode (15) is formed of two adjacent conductive layers (16, 17) spaced from one another by a gap (11). The floating electrode includes a second dielectric (19) having a conductive layer (13) thereon opposite the two layers (16), (17). A first capacitance is formed by the first conductive layer (16) of the center margin electrode, the dielectric (12) and the floating electrode (13). A second capacitance is formed by the second conductive layer (17) of the center margin electrode, the dielectric (12) and floating electrode (13). The common floating electrode connects the two capacitances in series which creates a potential between the conductive layers (16, 17) across the gap (11) of the center margin electrode.
Metallized film capacitors are known to have a "self-healing" capability, whereby short circuits between the conducting layers on opposite sides of the dielectric caused by a fault in the dielectric will be "cleared" or "self-healed". Self healing occurs in a series section capacitor when a defect in the dielectric (12) causes the center margin electrode (15) to short to the floating electrode (13) causing the metal of the electrodes to oxidize. The oxidation acts as an insulator which heals the short. If the metal forming the center margin electrode, however, is too thick, the heat required to oxidize and/or vaporize the metal will cause the center margin electrode to melt through the dielectric to the floating electrode creating an internal short which will not heal.
A problem associated with previous metallized film series section capacitors arises at the gap of the center margin electrode. The potential created across the gap causes the metal at the borders of the center margin electrode bordering the gap to oxidize. This oxidation reduces the effective area of the conductive layers (16) and (17) causing the overall capacitance of the capacitor to decrease.